1. Field of Invention
This invention relates to a device for delivering fluids, preferably for delivering medical fluids, comprising at least one pump designed as a displacement pump and at least one control unit controlling the operation of the pump.
2. Description of the Prior Art
The use of displacement pumps is known in many different fields of application, such as in the field of medical engineering. Dialysis machines are known, for instance, in which dialysis fluid is prepared with the aid of displacement pumps. In such dialysis machines, the dialysis fluid is prepared from ultrapure water (RO water) and from one or more concentrates containing the required electrolytes and other constituents. The ultrapure water and the concentrates are mixed in a suitable ratio. The mixing ratio between ultrapure water and concentrate can for instance be about 34:1 or 44:1. Other ratios are of course also conceivable.
The delivery of concentrate frequently is performed by means of diaphragm pumps operating with a defined stroke volume. A dialysis machine with a diaphragm pump for delivering the concentrate is known from DE 28 38 414. In the machine known from this document, ultrapure water is mixed with concentrate upon passing through a pressure-reducing valve. The delivery of the concentrate is effected in synchronism with the actuation of the balancing device, so that per cycle of the balancing device one cycle or a defined number of cycles of the pump is each performed.
In prior art dialysis machines, the dialysis fluid frequently is prepared in discrete portions, i.e. in the form of so-called batches. The quantity of the concentrate to be delivered is not constant, but can vary from concentrate to concentrate or in dependence on the treatment to be performed. Thus, with different concentrates it frequently occurs that the same need not all be delivered in the same quantity, but that the delivery volumes can differ considerably. It is conceivable, for instance, that with a batch volume of 30 ml the range of the delivery volumes extends from 200 μl to 2200 μl.
Displacement pumps involve the problem that the accuracy of the delivery depends on the delivery volume. A particularly high accuracy is obtained when for one batch a stroke with a delivery volume is used, which corresponds to the maximum possible delivery volume per stroke or at least approximately corresponds to the same.
An example of a diaphragm pump known from the prior art is shown in FIG. 2 in a longitudinal section. The pump includes a diaphragm 10, which defines the delivery space of the pump on one side and whose movement leads to a delivery of fluid. The movement of the diaphragm 10 is realized by an eccentric drive 20 with stepper motor. The connecting rod 22 is connected with the diaphragm 10 and translates the movement of the eccentric drive 20 into a movement of the diaphragm 10. Reference numeral 30 designates the pump housing, which is composed of the drive housing and the pump head screwed to the same, which also includes the valves controlling the filling and evacuation of the delivery space. In FIG. 2, one of the valves is designated with the reference numeral 40.
FIG. 1 shows a pump characteristic of the pump shown in FIG. 2. In the embodiment illustrated here, the pump is dimensioned such that it can deliver a maximum of about 1200 μl per stroke. In this pump, the required accuracy of 1% starting with delivery volumes of about 600 μl can easily be realized by an individual calibration of the pump. As regards accuracy, however, the lower range of 200 μl to 600 μl turns out to be more problematic, the range between 200 μl and 300 μl being particularly important for the use of dry concentrate. Due to the dependence on the input and output pressure and on the existing time drift, accuracy neither can be achieved in this range by a better calibration.
This results in the problem that in particular for small delivery volumes special pumps would be required to ensure the required accuracy, which would involve the disadvantage of a comparatively large amount of apparatus.
A dialysis machine with a plurality of concentrate pumps is known for instance from EP 160 272. One of the concentrate pumps is intended to provide a basic dialysis fluid, whose ion concentration is absolutely necessary for the treatment. The second concentrate pump effects the adaptation of the ion concentration of the dialysis fluid to the individual requirements, which can vary from patient to patient.
From WO 99/30756 a displacement pump of a dialysis machine is known, which delivers concentrate from a container to a mixing point. As soon as the filling level in the container falls below a specified level, the container is filled with concentrate beyond the specified level by a delivery pump of the same construction. In this way, it is ensured that upon operation of the pump evacuating the container, there is always emitted a signal, which is generated when the level falls below the specified filling level. The signal activates the pump filling the container.